Gypsum (calcium sulfate dihydrate or hemihydrate) is a well-known material used in making industrial and building products, especially gypsum wallboard. It is a plentiful and generally inexpensive raw material, which, through a process of hydration may turn into slurry, which may be cast, molded or otherwise formed to useful shapes, and dried to a final product. Gypsum wallboard generally consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets, and is used largely for interior wall and ceiling applications.
However, gypsum products, e.g., wallboard, tile, block, casts and the like, have relatively little resistance to water. When ordinary gypsum wallboard, for example, is immersed in water, the board may quickly absorb a considerable amount of water, lose a great deal of its strength, and distort or swell in different places. Many attempts have been made in the past to improve the water resistance of gypsum products by adding waterproofing materials within the gypsum slurry. The most common waterproofing material in use today is a hydrophobic emulsion, usually an emulsion of wax, paraffin, asphalt or a silicone compound, e.g., silanes and siloxanes.
It is important to define clearly the difference between an emulsion and a suspension. An emulsion is defined as a dispersion of one liquid in a second immiscible liquid. One well-known example of an emulsion is milk; another is certain kinds of paint. A suspension, on the other hand, is defined as a dispersion of fine solid or liquid particles in a fluid (liquid or gas), the particles being supported by buoyancy.
Some emulsified silicone compositions used to make gypsum products water repellent may contain an emulsion of polysiloxane and some parts by weight of a liquid suspension of colloidal silica and other parts by weight of an emulsifier stabilizer. However, the silicone composition that is actually added to the gypsum slurry is an emulsion, not a suspension.
Other methods have been proposed in the prior art to improve the water resistance of gypsum products by coating the finished gypsum product with water resistant films or coatings. Another method involves spraying molten paraffin, wax, asphalt, and the like, into the aqueous gypsum slurry.